In wavelength division multiplexing (WDM), data is transmitted simultaneously over a single fiber on multiple carrier signals that have different center wavelengths. The number of carrier signals that can be carried on a single optical fiber is a function of the linewidth of each carrier signal and how accurately the center wavelength of each carrier signal can be controlled. To accommodate the increasing demand for optical communications bandwidth, the number of optical channels per fiber has increased substantially, resulting in smaller wavelength spacing between optical channels in dense wavelength division multiplexed (DWDM) systems.
In order to keep multiple carrier signals from interfering with each other and to maximize the number of carrier signals that can be multiplexed into a fiber, it is necessary to accurately control the wavelength of the carrier signals that are output by a laser into an optical fiber. One type of laser that can be used to generate carrier signals in DWDM systems is an external cavity laser. FIG. 1 depicts an example of a prior art external cavity laser 100. The external cavity laser 100 includes an optical gain medium 110, a first lens 130, a diffraction grating 140, a partially reflective mirror 120, and a second lens 150. When activated, the external cavity laser 100 produces an optical output 170. In known external cavity lasers, the cavity between the gain medium 110 and the diffraction grating 140 is exposed to an uncontrolled atmosphere. Such an uncontrolled atmosphere may possess unwanted wavelength-dependent absorptive characteristics that reduce the performance of the laser. For example, an atmosphere of air includes water vapor that can absorb light in varying degrees resulting in output power changes at wavelengths that the water absorbs. Additionally, using known external cavity lasers, calibration of the output wavelength of the laser is difficult to achieve.
In view of the desire to multiplex more carrier signals into a single fiber, what is needed is an external cavity laser with a stable center wavelength and a simple and accurate technique for calibrating an external cavity laser.